Time synchronization system, satellite system applied to the time synchronization system, ground system applied in the time synchronization system, time synchronization method and a computer-readable recording medium with a program

ABSTRACT

In a satellite system, a time frame to be used for establishing a time correlation between the satellite system and a ground station is inserted between the transmission frames at an arbitrary timing. The ground station computes the time at which the data was generated in the satellite system from this time frame and establishes a time correlation with the satellite time by using only this time frame.

FIELD OF THE INVENTION

The present invention relates to a time synchronization technology usinga satellite. More specifically, this invention relates to a timesynchronization system for synchronizing the time in the satellitesystem with the time in the ground system, a satellite system applied inthe time synchronization system, a ground system applied in the timesynchronization system, a time synchronization method, and acomputer-readable recording medium with a program for making a computerexecute the method.

BACKGROUND OF THE INVENTION

When astronomical events are observed using a remote system such as asatellite, it is required to synchronize the time in the satellitesystem with that of the ground time in order to know the time ofgeneration of observed events. With recent advance in the field ofcomputers, high speed data processing has become possible in thesatellite system, and also an advanced protocol such as packet telemetrybased on recommendation by CCSDS (Consultative Committee for Space DataSystems) for communications between a satellite system and a groundstation are used.

Because of the necessity of data processing in a satellite or employmentof packet telemetry, ambiguous delay in time interval is generated fromthe time when data is acquired until the time when the data istransmitted to the ground. Therefore, it is difficult to estimate thetime of generation of an event from a time when the data is received atthe ground station. On the other hand, it is required to know theaccurate time of generation of data while realizing a high speed dataprocessing in the satellite system or an advanced protocol such aspacket telemetry. Especially in astronomical observation, in order toverify the result of observation to that acquired by other satellites orthat obtained on the ground, precision in time measurement of the orderof microseconds is required.

In the conventional technology, observed data is sampled according tothe timing generated by an apparatus for controlling the timing ofoperations of the entire satellite system. The data is inserted at afixed location in the transfer frame and the time at which the data wasgenerated is determined from the time when the transfer frame isreceived. On the other hand, a system in which request for the currenttime is made when required to a device which controls the system time,or a system in which standard time is determined by using data inputtime into a device generating a transfer frame is employed in asatellite system employing the packet telemetry therein.

In recent years, time is determined by using the GPS (Global PositioningSystem). FIG. 15 shows the commonly used GPS system. The system shown inFIG. 15 is a satellite system in which an orbiting satellite 105acquires the signals, namely the observation data, from four GPSsatellites 101 to 104. In this satellite system, the satellite 105acquires apparent distances between itself and the GPS satellites 101 to104 based on the acquired observation data, and obtains four unknownparameters i.e. its own position (x, y, z) and the difference betweenits own time and the time in the GPS satellites. With this method, anaccurate time can be acquired in the satellite 105.

In the satellite system described above, in association with the advancein the computer technology, sophisticated processing such as datacompression or data extraction has become possible, so that data lengthof the observed data or the like changes, and sometimes waste ofresources occurs in data transfer when a fixed data format like that inthe conventional technology is used. Efforts have been made in order toimprove the efficiency in data transmission by employing packettelemetry such as CCSDS.

However, in the satellite system described above, because thecomplicated data processing such as the packet telemetry like CCSDS isemployed, a delay is generated until the acquired data is packetized, orambiguous delay is generated until a packet including data is actuallyedited into the transfer frame. Further, fluctuations in time fromgeneration up to transfer of data to the ground station becomes larger,so that it is difficult to decide the time at which the data wasgenerated from the time at which the data is received.

In a system in which time is required to be acquired by the a devicewhich manages the time in the satellite, non-uniformity of around acouple of tens of microseconds is included in association withrealization of a protocol for acquiring time, so that an error which isnot desirable in a system requiring accurate time may be generated.

High precision time determination can be realized with GPS shown in FIG.15 having been employed and becoming popular in recent years. However,the system configuration is very complicated. Further, in the exampleshown in FIG. 15, because the satellite 105 itself rotates around theEarth at a high speed a Doppler shift is generated. This Doppler shiftmakes the use of GPS on the ground difficult.

SUMMARY OF THE INVENTION

To solve the problems as described above in the conventional technology,it is an object of the present invention to provide a timesynchronization system capable of determining time of generation of datawith high precision and also with a simple configuration. It is also anobject of the present invention to provide, a satellite system appliedin the time synchronization system, a ground system applied in the timesynchronization system, a time synchronization method, and acomputer-readable recording medium with program for making a computerexecute the method recorded therein.

In the present invention, the satellite system inserts a time frame tobe used for establishing a time correlation between the satellite systemand the ground system between the transmission frames at an arbitrarytiming, and the ground system acquires the time of generation of data inthe satellite system from this time frame. Thus, a time correlationbetween the time in the satellite and that on the ground can beestablished only by using the time frame, so that time of generation ofdata can precisely be determined with a simple configuration.

Further, the transmission frame is generated by packetizing the datagenerated in the satellite system, so that even complicated dataprocessing can be executed.

Further, the satellite system generates the transmission frame utilizingthe observation data generated in a plurality of equipments mountedthereon, so that the ground system can acquire a result of observationin the satellite.

Further, the satellite system distributes a time clock to each of theequipment and synchronizes the internal time in satellite (satellitetime) with of the internal time in each of the equipments, so that acentralized time management in the satellite system can be realized.

Further, the satellite system appends the satellite time to the datagenerated in each of the equipment, so that time management of data canbe realized in the satellite system.

Further, the satellite system generates the satellite time by countingclocks which are synchronized to a bit rate of the transmission frames,and set an entire portion of the satellite time below the time requiredfor transmission of one frame to zero at the head of a transmissionframe, so that a fraction of satellite time is eliminated and precisionin time synchronization can be improved.

Further, in the satellite system, satellite time is corrected dependingupon the changes in the temperature, so that time error due totemperature conditions inside the satellite system can be suppressed.

Further, the satellite system inserts satellite time at two differenttimings between the transmission frames, while the ground systemacquires an average frequency of satellite time from the time intervalbetween these two satellite times inserted between the transmissionframes and a time interval between the ground times corresponding tothese two satellite times. Then, the ground system corrects the time ofgeneration of data in the satellite from the above average frequency andthe amount of drift from the official frequency of the satellite time.With this, error in time progress in the satellite system and that inthe ground system can be corrected, which makes it possible to acquireaccurate time of generation of data in the satellite system.

Further, the satellite system estimates the amount of change in thefrequency to be used for acquiring the satellite time according to achange of the internal temperature, and the ground system corrects thetime of generation of data in the satellite system taking into accountthe estimated change in the frequency. Thus, the ground system canacquire the accurate time of generation of the data by taking intoaccount the satellite time in the satellite system.

In the present invention, a time frame to be used for establishing atime correlation between the satellite system and the ground system isgenerated, and this time frame is inserted, when the frames aretransmitted, between the transmission frames at an arbitrary timing togenerated a transfer frame. Therefore, time correlation between thesatellite time and the ground time can be established in the groundsystem utilizing this time frame.

Further, a convolution processor is provided for generating the transferframes by packetizing the generated data, so that even complicated dataprocessing can be executed.

In the present invention, the transmission frame is generated utilizingthe observation data generated in a plurality of equipments mounted onthe satellite system, so that a result of observation by the satellitesystem can be provided to the ground system.

Further, satellite time preserved in the satellite system issynchronized with the internal time in each of the equipment bydistributing time clocks to each equipment, so that monolithic timemanagement can be realized in the satellite system.

Further, satellite time is appended to data generated in each of theequipment, so that time management for each data can be realized in thesatellite system.

Further, satellite time is generated by counting clocks eachsynchronized to a bit rate of the transfer frames, and an entire portionof satellite time below the time required for transfer of one frame isset to zero at the head of the transfer frames, so that a fraction ofsatellite time is eliminated and precision in synchronization can beimproved.

Further, satellite time is corrected according to the changes in thetemperature, so that a time error due to temperature conditions insidethe satellite system can be suppressed.

In the present invention, time of generation of data in the satellitesystem is acquired according to a time frame inserted between receivedframes to be used for establishing a time correlation between thesatellite system and the ground system. Thus, time correlation betweenthe satellite system and ground system can be realized according to thetime frame obtained from the satellite system, so that time ofgeneration of data can precisely be determined with a simpleconfiguration.

Further, an average frequency of satellite time is acquired from thetime interval between two different satellite times and the timeinterval between the ground times corresponding to the two satellitetimes, and time of generation of data in the satellite system iscorrected from this average frequency as well as from the amount ofdrift from then official frequency of satellite time. With this, errorbetween time progress in the satellite system and that in the groundsystem can be corrected, so that an accurate time of generation of datain the satellite system can be acquired.

Further, time of generation of data in the satellite system is correctedby taking into account the change in the frequency estimated in thesatellite system, so that accurate time of generation of data can beacquired by taking into account the satellite time in the satellitesystem.

In the present invention, a time frame to be used for establishing atime correlation between the satellite system and the ground system isgenerated, and a transfer frame is generated, when frames aretransmitted, by inserting the generated time frame at an arbitrarytiming between the transmission frame. Thus, time correlation betweenthe satellite time and ground time can be established based on this timeframe, so that accurate time of generation of data can precisely bedetermined with a simple configuration.

In the present invention, when receiving the frames, time of generationof data in the satellite system is acquired from the time frame insertedbetween the received frames to be used for establishing a timecorrelation between the satellite system and the ground system, and datais analyzed according to the time of generation of data. Then a timecorrelation between the satellite system and ground system can beestablished according to the time frame obtained from the satellitesystem and data can be analyzed using an accurate time, so that a resultof data analysis can be acquired based on accurate time in the satellitesystem.

In the present invention, processing for generation of a time frame tobe used for establishing a time correlation between the satellite timeand the ground system is executed, and then a transfer frame isgenerated by inserting, when transmitting the frames, this time frame atan arbitrary timing between the transmission frames. Thus, timecorrelation between the satellite system and the ground system can beestablished according to this time frame by using a computer program, sothat time of generation of data can precisely be determined with asimple configuration.

In the present invention, when receiving the frames, time of generationof data in the satellite system is computed according to the time frameinserted between the received frames, to be used for establishing a timecorrelation between the satellite system and the ground system, and datais analyzed according to the computed time of generation of data. Thenthe data can be analyzed according to an accurate time by establishing atime correlation between the satellite time and the ground timeaccording to the time frame obtained from the satellite system using acomputer program, so that a result of analysis based on an accurate timecan be acquired.

Other objects and features of this invention will become understood fromthe following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a general configuration of a timesynchronization system according to one embodiment of the presentinvention;

FIG. 2 shows how a transmission frame is generated in a satellitesystem;

FIG. 3 explains the CCSDS packet format;

FIG. 4 explains the MPDU format;

FIG. 5 explains the VCDU format;

FIG. 6 explains the transmission frame format;

FIG. 7 is a flow chart showing operations in the satellite system;

FIG. 8 is a flow chart showing receiving operation in a ground station;

FIG. 9 explains how the time of generation of data is computed;

FIG. 10 is a view showing operations for generating data and appendingtime to the generated data;

FIG. 11 is a block view showing synchronous configuration in thesatellite system;

FIG. 12 shows an example of time synchronization in the satellitesystem;

FIG. 13 is a view showing high precision time appending;

FIG. 14 is a graph showing relation between a time correction value andsatellite time; and

FIG. 15 is a block diagram showing a commonly used GPS system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed description is made hereinafter for preferred embodiments ofthe time synchronization system, a satellite system applied in the timesynchronization system, a ground system applied in the timesynchronization system, a time synchronization method, and a computerreadable recording medium with a program for making a computer executethe method recorded therein.

At first, configuration is described. FIG. 1 is a block diagram showinga time synchronization system according to one embodiment of the presentinvention. For instance, as shown in FIG. 1, the time synchronizationsystem according to the present invention comprises a satellite system1, a ground station 2, and ground system equipment data analyzingdevices B1 to Bn.

The satellite system 1 comprises equipment A1, A2, . . . An (n: naturalnumber) which generate the observation data, a transfer frame generatingdevice 10 for generating a packetized transfer frame containing theobservation data and the satellite system time, and a transmitter 20 fortransmitting the generated transfer frame to the ground station 2.

The transfer frame generating device 10 comprises a convolutionprocessor 11, a timing generator 12, a time frame generator 13, a frameselector, a transfer frame generator 15, and a time error estimator 16.The convolution processor 11 generates a flow of data in order togenerate a transfer frame. The timing generator 12 generates a timingfor controlling the timing of the entire satellite system 1.

The time frame generator 13 converts the time in the system into a framebased on the timing generated by the timing generator 12. The frameselector 14 selects an ordinary data frame or a time frame. The transferframe generator 15 generates a transfer frame (VCDU) to be finallytransferred to the transmitter 20. The time error estimator 16 estimatesan error caused due to a change in the temperature and included in thetiming generator 12.

The ground system 2 comprises a receiver 30, a time appending section40, a depacketizing section 50, and a data generation time computingsection 60. The receiver 30 receives the transmission frame transmittedfrom the satellite system 1. The time appending section 40 appends thetime at which the transfer frame is received (Referred to as receivingtime hereinafter) to the received transfer frame. The depacketizingsection 50 depacketizes the received transfer frame received thereby todata for each equipment. The data generation time computing section 60computes the time at which the data was generated (Referred to as datageneration time hereinafter) with respect to the data for each equipmentacquired by depacketizing with the depacketizing section 50 according toa time frame inserted by the satellite system 1 into the transfer frameand the receiving time appended in the receiving time appending section40.

The equipment data analyzing devices B1 to Bn correspond to theequipment Al to An respectively, and analyze the observation datagenerated in the respective equipment.

Next, a format of data applied in this time synchronization system isexplained. FIG. 2 shows how a transfer frame is generated in thesatellite system 1. FIG. 3 shows the CCSDS packet format, FIG. 4 showsthe M_PDU (Multiplexing Protocol Data Unit) format, FIG. 5 shows the VCD(Virtual Channel Data Unit) format, and FIG. 6 shows the transfer frameformat.

In this time synchronization system, the packet system based on theCCSDS recommendation is employed as a telemetry system. The transferframe generating device 10 generates the format shown in FIG. 6, asshown in FIG. 2, by forming the M_PDU format shown in FIG. 4 generatedby multiplexing the CCSDS packet shown in FIG. 3 and then forming theVCDU format shown in FIG. 5. The CCSDS packet comprises, as shown inFIG. 3, a 6-octet primary header, a secondary header (32 bits) and userdata set within 4 octet to 65536 octet.

The primary header comprises areas for a version (3 bits), a type (1bit), a secondary header flag (1 bit), an application identifier (11bits), a sequence flag (2 bits), a sequence count (14 bits), and apacket length (16 bits).

A user data area of the CCSDS packet has a variable length, andinformation regarding its length is stored in the packet length area.Data indicating whether the secondary header is present or not is storedin the secondary header flag area. The application identifier areastores therein the ID appended to the CCSDS packets depending upon theirtype. By identifying this application identifier, it is possible todetermine a device that generated the packet or the type of the packetor the like. This application identifier is used for synchronizing thetime, and information regarding the satellite time for timesynchronization is stored in the CCSDS packet.

The sequence count stored in the sequence count area is a counterappended to each CCSDS packet having the same application identifier.Whether any packet is missing or not can be identified by checkingcontinuity of this sequence count. The sequence flag stored in thesequence flag area indicates whether data in the CCSDS packet iscomplete within one packet or the data is formed with a combination ofpackets.

As shown in FIG. 4, the M_PDU uses a format comprising a spare, a firstheader pointer, and a packet zone. As shown in FIG. 5, the VCDU uses aformat comprising a version, a spacecraft ID, a VCID, a VCDU counter, areplay flag, a spare, a VCDU data unit zone, an OPER. CTRL. field, and aCDVU error CTRL. field. As shown in FIG. 6, the transfer frame uses aformat comprising a ASM (Attached Sync. Marker) which is a framesynchronization code, and the VCDU described above.

The transfer frame format shown in FIG. 6 is a format of the datafinally outputted from the satellite. This transfer frame format has theVCDU shown in FIG. 5. The VCDU data unit zone shown in FIG. 5corresponds to the M_PDU format shown in FIG. 4. The VCID indicates thetype of VCDU.

The VCID is provided for synchronizing the time, and includes a CCSDSpacket for time synchronization. Stored in the CCSDS packet isinformation regarding the satellite time at the time when a headerposition of ASM (Refer to FIG. 6) to which the VCDU belongs) isoutputted. In the ground station 2, receiving is performed with thetransfer frame format shown in FIG. 6, and when the VCID for timesynchronization is detected, receiving time of the ASM to which the VCIDbelongs is recorded.

When the inside of VCDU is decoded, a CCSDS packet having an APIDdedicated for time synchronization is included in this VCDU, and thesatellite time when the ASM is generated is stored in this CCDSD packet.Time correlation between the satellite system and the ground system canbe established from the satellite time stored in the CCSDS packet andthe receiving time on the ground.

Operations of the entire system is explained below. FIG. 7 is a flowchart showing operations in the satellite system 1. Operations in theground station 2 are also described together with this description. Thesatellite system 1 operates in synchronism with the timing outputtedfrom the timing generator 12. Namely, time is counted up each time allof the operations shown in FIG. 7 are finished (step S1). The equipmentA1 to An generate data at arbitrary timing. This data contains the datageneration time based on the time information received from the timinggenerator 12. In the convolution processor 11, convolution processing iscarried out with respect to the data generated by the equipment A1 toAn, and edited with a format of a transfer format (VCDU) with a fixedlength. Generally the equipment A1 to An send the output data to thetransmitter 20.

The time frame generator 13 generates a time frame at a required timing,and sends the time frame to the frame selecting section 14. This timingwill be called as a distribution timing. When a request for output of atime frame is issued from the timing generator 12 (step S2), the frameselecting section 14 preferentially treats this request, and then thetime frame is sent to the transfer frame generator 15 in such a way thatthe contents of the time frame and the actual time match with eachother. In the transfer frame generator 15, synchronization code, errorcorrection code or the like are appended to the frame, and the frame isoutputted to the transmitter 20. The time is distributed as describedhere (step S3). If it is not the time to output VCDU, system control isreturned to step S1 (step S4).

When the transfer frame is received by the receiver 30 of the groundstation, time at which the frame is received is appended to the receivedtransfer frame in order to establish accurate time correlation betweenthe transfer frame and ground time. Thus, a time correlation between thetime in the satellite system 1 included in the time frame and the actualtime can be established.

If it is determined in step S4 that it is time to output VCDU, then itis determined in step S5 as to whether it is time to output VCDU of timeor not. If it is determined that it is time to output VCDU of time (stepS5, “Yes”), VCDU of time is edited (step S6), and the edited VCDU oftime is outputted (step S8). On the other hand, if it is not the time tooutput VCDU of time (step S5, “No”), VCDU editing of equipment data isperformed (step S7), and the edited VCDU of equipment data is outputted(step S8).

Detailed description is made for a receiving operation in the groundstation 2. FIG. 8 is a flow chart of the receiving operation in theground station 2. A frame is received (step S11) and data regarding thereceiving time is appended to this frame (step S12). The time appendedhere is the time counted in the ground station 2.

When the receiving time is appended to the received frame as describedabove, the received frame is subjected to depacketize processing forsubdividing the received frame into packets (step S13). When the timepacket included in the time frame is confirmed (step S14, “Yes”), thesatellite time stored in the time packet and the receiving time of theframe are recorded in a correlated form (step S15). Then the processingis returned to the above described step S11. On the other hand, when thetime packet cannot be confirmed (step S14, “No”), data generation timeis computed (step S16). Then the processing is returned to the abovedescribed step S11.

Next, a method for computing the data generation time will be explained.FIG. 9 explains how the data generation time is computed. In FIG. 9,STT0 and STT1 indicate satellite times when a time frame is generated,and STD indicates satellite time when the data is generated. GTT0 andGTT1 indicate ground time when the time frames (generated at time STT0and STT1 respectively) are received respectively, and GTD indicatesground time corresponding to the satellite time STD.

Of the six times, namely the satellite times STT0, STT1, STD, and groundtimes GTT0, GTT1, GTD, all the times other than the ground time GTD areknown. Accordingly, computation of data generation time means thecomputation of this ground time GTD.

In the example shown in FIG. 9, the ground time GTD can be obtained fromthe following expression (1) or (2):

GTD=GTT 0+(STD−STT 0)  (1)

GTD=GTT 1+(STD−STT 1)  (2)

Next description is made for time appending in the satellite system 1.FIG. 10 is a view explaining generation of data and the operation forappending time to this data. In this figure, t1, t2, and t3 are timingsin the satellite system when data 1, data 2, and data 3 are generatedrespectively, namely the satellite times. Data 1 is stored in thetransfer frame 1 and outputted. Data 2 is convoluted in the transferframe 1 and transfer frame 3 and then are outputted. Data 3 is stored inthe transfer frame 3 and then is outputted. When the data is outputted,the data 1, 2, 3 are transmitted to the ground station 2 along with thetiming t1, t2, t3 indicating the data generation time included therein.

The transfer frame 2 is a time frame, and is transmitted to the groundafter including in it a header time t4 of the transfer frame. FT0, FT1,FT2, and FT3 are ground times when the transfer frames 1, 2, and 3 arereceived respectively. The time t1 when the data 1 is generated and thetime when the transfer frame with the data 1 stored therein istransmitted are uncertain depending on the situation. Therefore, thetime when the data 1 is generated can not be correlated from the groundtime FT1. However, because the transfer frame 2 which is a transferframe dedicated for time transmission is included, a time correlationbetween the satellite time t4 and the ground time FT2 can be establishedfrom the received time FT2. By calculating reversely from this time FT2,it is possible to determined which of the ground time corresponds to thesatellite time t1.

Time set in the time frame is the header time of the frame. Forinstance, assuming that a transmission rate is 1/1048576 bps (=1/2²⁰bps) and further assuming that one frame contains 8192 bits (=2¹³ bits),then 128 (=2⁷) transfer frames are generated per second. Assuming that atime unit in the satellite system is 1/262144 (=1/2¹⁸) sec and the timerlength is 32 bits, then satellite time at the header of each transferframe becomes 00000800h, 00001000h, 00001800h, . . . . Assuming that onetime frame is inserted after every 64 seconds, then a time frame may begenerated and outputted at the satellite time of 01000000h, 02000000h,03000000h, . . . .

It is not necessary to output this time frame periodically. The timeframe may be outputted only in response to a request from the groundstation 2. It is important to match the header time of this time framewith the time information included therein. As in the example describedhere, by employing the configuration in which the satellite time issynchronized with the transfer frame and at the same time a lower digitof time is zero at the header of the frame, satellite time at the firstrise position of the header bit can be identified. By identifying theground time at the first rise point in the ground station 2, even in acase of data transfer at a slow rate in which several tens ofmicroseconds are required for transferring 1-bit data, accuratesynchronization between the ground time and satellite time can beachieved.

Next, a method for synchronizing the time among the equipment A1 to Anwill be explained. At first description is made for configuration forsynchronization. FIG. 11 is a block diagram showing configuration ofsynchronization of time between the transfer frame generating device 10and the equipment A1 to An. FIG. 12 shows an example of timesynchronization between the transfer frame generating device 10 and theequipment A1 to An. In FIG. 11, the transfer frame generating device 10comprises a upper-digit counter 31, a lower-digit counter 32, and anoutput circuit 33. On the other hand, each of the equipment A1 to An hasa common configuration comprising an input circuit 41, a fixed-valuegenerator 42, an upper-digit counter 43, and a lower-digit counter 44.The upper-digit counters 31 and 43 indicate a time above second, whilethe lower-digit counters 32 and 44 indicate the fraction of the second.

In order to precisely synchronize the time in the equipment A1 to An ona satellite with the time in the transfer frame time, it is necessary todistribute a clock from the timing generator 12. Time information havingthe same timing as that in the transfer frame can be generated by usingthe distributed clock in each of the equipment from the time informationreceived from the timing generator 12 as a starting point. When data forthe equipment A1 to An is generated, this time information is appendedto the data as a data generation time.

As shown in FIG. 12, it is assumed herein that the transfer framegenerating device 10 in the satellite system 1 transmits a 1 MHz (2²⁰)clock to each of the equipment A1 to An from the upper-digit counter 31and the lower-digit counter 32. The equipment A1 to An make the countersoperate according to this clock. The transfer frame generating device 10transmits a time information through the upper-digit counter 31 at everysecond, namely when the lower-digit counter 32 has become zero.

When each of the equipment A1 to An in the receiving side receives thistime information through the input circuit 41, the equipment stores thistime information in the upper-digit counter 43, and also stores afixed-value from the fixed value generator 42 in the lower-digit counter44. It should be noted that data transfer between the transfer framegenerating device 10 and each of the equipment A1 to An is performedbased on the enable state of the output circuit 33. The timing when theoutput circuit 33 is enabled is generated at n second, n+1 second, n+2second, . . . of the satellite time.

In this example, as time information is stored in 24 bits after a borderbetween seconds, so that 00018h is stored and then counting-up iscontinued. With this method, an error in time synchronization in thesatellite system 1, namely between the transfer frame generating device10 and each of the equipment A1 to An can be suppressed to the level ofthe delay in the interface element. Therefore, time synchronization withan error of only around several tens of nanoseconds, or several hundredsof nanoseconds can be achieved.

A separate ID flag may be provided to the transfer frame generatingdevice 10 and to each of the equipment A1 to An in data in the interfaceso that the information other than the time information can betransmitted through the same interface. With the configuration in whichtime information is outputted periodically, even when the timeinformation and the information other than the time information areoutputted at the same timing and in case the time information can not beoutputted, because the clocks are transmitted, an internal counter inthe receiving side continues to operate and time synchronicity can bemaintained.

As described above, with this embodiment, it is possible to removeuncertain elements for deciding time such as residing of packet data inthe satellite system or drift of satellite time, and timesynchronization between the satellite system 1 and the ground station 2can be achieved with a relatively simple configuration.

By fixing the time information when the data was generated, thenecessity of being aware of the time required for data processing orpacket generation is eliminated, so that independency of equipmentsinvolved in data generation is improved, and with this feature systemconstruction becomes easier, and also decision of time of generation ofobservation data requiring long time and sophisticated processing caneasily be made.

In the satellite system 1, a time frame to be used for establishing timecorrelation between the satellite system 1 and the ground station 2 isinserted between transmission frames at an arbitrary timing, and theground station 2 acquires time of generation of data inside thesatellite system according to the received time frame. With this, timecorrelation between the satellite time and the ground time can beestablished only by using the time frame, so that it is possible todetermine the data generation time with a simple configuration.

Further, because the transmission frame is generated by packetizing thedata generated in the satellite system 1 with the telemetry formationbased on recommendation by CCSDS, complicated data processing can beexecuted.

Further, transmission frame is generated in the satellite system 1 byusing the observation data generated from a plurality of equipments A1to A1 mounted thereon, so that it is possible for the ground system 2 toacquire a result of observation by the satellite.

Further, synchronization between the satellite time and the internaltime of each of the equipments A1 to An is achieved by distributing timeclocks to the equipments A1 to An in the in the satellite system 1, sothat monolithic time management can be realized in the satellite system1.

Further, in the satellite system 1, satellite time is appended to datagenerated in each of the equipments A1 to En, so that time managementcan be realized for each data in the satellite system.

In the satellite system 1, satellite time is generated by countingclocks each synchronized to a bit rate of the transmission frames, and aportion of the satellite time below a time required for transmitting oneframe is set to zero at the head of each transmission frame, so that afraction of satellite time is eliminated, which insures precisesynchronization.

In the satellite system 1, satellite time may be corrected according tochange in temperature, and in this case a time error can be suppressedto an allowable level according to temperature conditions in thesatellite system 1, and an operation for correcting satellite timeaccording to change of temperature in the satellite system 1 is notrequired in the ground station 2.

Next, description is made for variants of the present invention. Atfirst description is made for realization of high precision timecorrelation. FIG. 13 is a view showing high precision time correlation.The method of realizing time correlation between satellite time andground time shown in FIG. 10 uses only one time frame. However,precision can be increased by using two time frames. As shown in FIG.13, it is assumed herein that header times of the two time frames in thesatellite side are ST1 and ST5, and respective receiving times are GT1and GT5.

A problem will not arise if the satellite time is correct. However, inreality, because the time is generated using, for instance, a quartzoscillator, there is generally generated an error of the order ofseveral ppm to several tens of as compared to the official time. Whenthe oscillation frequency of the quartz oscillator is completely thesame as the official value, then ST5 and ST1, and GT5 and GT1 will haveexactly the same values. However, in reality, differences in theoscillation frequency causes the timers to become faster or slower withrespect to the actual time.

In order to suppress such a time error as much as possible, datageneration time in a satellite can be determined by computing an averageoscillation frequency Fa, of the timing generator 12 which generates thesatellite time, using the following equation (3):

Fa=(ST 5−ST 1)/(GT 5−ST 1)  (3)

An orbiting satellite generally makes a one rotation around the Earth inaround 90 minutes. In such a case, for instance, by deciding GT1 and ST1in one rotation from the communicating with the ground station and thendeciding GT5 and ST5 in the next rotation, average oscillation frequencyFa during this time period can be decided.

If data generation time obtained in the rear side of the Earth andrecorded in the data recorder or the like is ST3, time GT3 can bedecided by adjusting advance or delay at GT1 or GT5 to zero through thefollowing equation:

GT 3=(ST 3−ST 1)×Fa+GT 1  (4)

The above relationship can be rewritten as the following expression (5)with reference to FIG. 9.

GTD=GTT 0+(STT 1−STT 0)/(GTT 1−GTT 0)×(STD−STT 0)  (5)

Thus, two different satellite times are inserted between transmissionframes in the satellite system 1 at two different timings. In the groundstation 2, average frequency of satellite time is computed from a timeinterval between the two satellite times and from the time intervalbetween the ground times corresponding to the two satellite times. Then,data generation time in the satellite system is computed according tothe average frequency and the amount of drift from the officialfrequency of the satellite time. Accordingly, it becomes possible tocorrect the error due to non-matching time in the satellite system 1 andthat in the ground station 2, so that accurate time of generation ofdata which is generated in the satellite system can be acquired.

Further, by correcting a time error because of a change in theoscillation characteristics due to a change in the temperature in anquartz oscillator by means of the methods disclosed by the sameapplicant in Japanese Patent Laid-Open Publication No. HEI 4-319695 andin Japanese Patent Laid-Open Publication No. HEI 5-273365 time can bedecided more accurately. It should be noted that information regardingtime correction is not always available when the data is generated.However, in such a case, the precision can be improved by interpolatingthe information regarding time correction corresponding to the datageneration time.

FIG. 14 shows an example of such an interpolation. As shown in thisfigure, information regarding time correction Y1, Y2, and Y3 isavailable at the satellite times st1, st2, st3 respectively. The datageneration time ste is included in the data, however, the data does nothave the information regarding time correction. In such a case,information regarding time correction Ye corresponding to the time stecan be obtained by interpolation from the satellite times andinformation regarding time correction by means of polynomialapproximation or the like. By using the Ye obtained in this way, acorrection processing for the data generation time ste is preferablycarried in order to determine an accurate correlation between groundtime and the data generation time ste.

The above relationship can be rewritten as the following expression (6)with reference to FIG. 9:

GDT=GTT 0+((STT 1−SE 1)−(STT 0−SE 0))/(GTT 1−GTT 0) ×((STD−SED)−(STT0−SE 0))  (6)

Herein SE0, SE1, and SED indicates an amount of error at timing STT0,STT1, and STD respectively.

In the satellite system 1, an amount of change in the frequency to beused for obtaining satellite time is estimated in response to a changein the internal temperature, and time of generation of data in thesatellite system 1 is corrected taking into account the estimated changein the frequency in the ground system. Therefore, accurate datageneration time can be acquired by taking into account the satellitetime in the satellite system 1 in the ground system.

In the present invention, the satellite system inserts a time frame tobe used for establishing a time correlation between the satellite systemand the ground system between the transmission frames at an arbitrarytiming, and the ground system acquires the time of generation of data inthe satellite system according to this time frame. Thus, a timecorrelation between the time in the satellite and that on the ground canbe established only by using the time frame, whereby there is providedthe advantage that time of generation of data can precisely bedetermined with a simple configuration.

Further, the transmission frame is generated by packetizing the datagenerated in the satellite system, whereby there is provided theadvantage that a time synchronizing system capable of executingcomplicated data processing can be obtained.

Further, the satellite system generates the transmission frame utilizingthe observation data generated in a plurality of equipments mountedthereon, whereby there is provided the advantage that it is possible toobtain a time synchronizing system capable of acquiring a result ofobservation in the satellite in the ground system.

Further, the satellite system distributes a time clock to each of theequipment and synchronizes the internal time in satellite with of theinternal time in each of the equipment, whereby there is provided theadvantage that it is possible to acquire a time synchronizing systemcapable of realizing a centralized time management in the satellitesystem.

Further, the satellite system appends the satellite time to the datagenerated in each of the equipment, whereby there is provided theadvantage that it is possible to obtain a time synchronizing systemcapable of realizing time management of data in the satellite system.

Further, the satellite system generates the satellite time by countingclocks each synchronized to a bit rate of a transmission frames, and setan entire portion of the satellite time below time required fortransmission of one frame to zero at the head of a transmission frame,whereby there is provided the advantage that it is possible to obtain atime synchronizing system capable of eliminating fraction of satellitetime and insuring precision in time synchronization.

Further, in the satellite system, satellite time is corrected dependingupon the changes in the temperature, whereby there is provided theadvantage that it is possible to obtain a time synchronizing systemcapable of suppressing error in time due to a change in the internaltemperature of the satellite system.

Further, the satellite system inserts satellite time at two differenttimings between the transmission frames, while the ground systemacquires an average frequency of satellite time from a time intervalbetween these two satellite times inserted between the transmissionframes and a time interval between the ground times corresponding tothese two satellite times. Then, the ground system corrects the time ofgeneration of data in the satellite from the above average frequency andthe amount of drift from the official frequency of the satellite time.Therefore, there is provided the advantage that it is possible to obtaina time synchronizing system capable of acquiring accurate time ofgeneration of data in the satellite system.

Further, the satellite system estimates the amount of change in thefrequency to be used for acquiring the satellite time according to achange of the internal temperature, and the ground system corrects thetime of generation of data in the satellite system taking into accountthe estimated change in the frequency. Therefore, there is provided theadvantage that it is possible to obtain a time synchronizing systemcapable of acquiring accurate time of generation of the data in theground system by taking into account the satellite time in the satellitesystem.

In the present invention, a time frame to be used for establishing atime correlation between the satellite system and the ground system isgenerated, and this time frame is inserted, when the frames aretransmitted, at an arbitrary timing between the transmission frames togenerated a transfer frame. Therefore, time correlation between thesatellite time and the ground time can be established in the groundsystem utilizing this time frame. Thus, there is provided the advantagethat it is possible to obtain a time synchronizing system capable ofdetermining data generation time precisely with a simple configuration.

Further, a convolution processor is provided for generating the transferframes by packetizing the generated data, whereby there is provided theadvantage that it is possible to obtain a time synchronizing systemcapable of executing complicated data processing.

With the present invention, the transmission frame is generatedutilizing the observation data generated in a plurality of equipmentsmounted on the satellite system, whereby there is provided the advantagethat it is possible to obtain a satellite system capable of providing aresult of observation by the satellite system to the ground system.

Further, satellite time preserved in the satellite system issynchronized with the internal time in each of the equipment bydistributing time clocks to each equipment, whereby there is providedthe advantage that it is possible to obtain a satellite system capableof realizing monolithic time management.

Further, satellite time is appended to data generated in each of theequipment, whereby there is provided the advantage that it is possibleto obtain a satellite system capable of realizing time management foreach data.

Further, satellite time is generated by counting clocks eachsynchronized to a bit rate of the transfer frames, and an entire portionof satellite time below the time required for transfer of one frame isset to zero at the head of the transfer frames. Therefore, there isprovided the advantage that it is possible to obtain a satellite systemcapable of eliminating a fraction of satellite time and insuringprecision in synchronicity.

Further, satellite time is corrected according to the changes in thetemperature, whereby there is provided the advantage that it is possibleto obtain a satellite system capable of suppressing a time error due tothe internal temperature conditions.

In the present invention, time of generation of data in the satellitesystem is acquired according to a time frame inserted between receivedframes to be used for establishing a time correlation between thesatellite system and the ground system. Thus, time correlation betweenthe satellite system and ground system can be realized according to thetime frame obtained from the satellite system. Therefore, there isprovided the advantage that it is possible to obtain a ground systemcapable of precisely determining data generation time with a simpleconfiguration.

Further, an average frequency of satellite time is acquired from thetime interval between two different satellite times and the timeinterval between the ground times corresponding to the two satellitetimes, and time of generation of data in the satellite system iscorrected from this average frequency as well as from the amount ofdrift from then official frequency of satellite time. With this, errorbetween time progress in the satellite system and that in the groundsystem can be corrected, whereby there is provided the advantage that itis possible to obtain a ground system capable of acquiring accurate timeof generation of data in the satellite system.

Further, time of generation of data in the satellite system is correctedby taking into account the change in the frequency estimated in thesatellite system, whereby there is provided the advantage that it ispossible to obtain a ground system capable of acquiring accurate time ofdata generation by taking into account the satellite time in thesatellite system.

In the present invention, a time frame to be used for establishing atime correlation between the satellite system and the ground system isgenerated, and a transfer frame is generated, when frames aretransmitted, by inserting the generated time frame at an arbitrarytiming between the transmission frame. Time correlation between thesatellite time and ground time can be established based on this timeframe. Thus, there is provided the advantage that it is possible toobtain a time synchronizing system capable of precisely decidingaccurate time of generation of data with a simple configuration.

In the present invention, when receiving the frames, time of generationof data in the satellite system is acquired according to the time frameinserted between the received frames to be used for establishing a timecorrelation between the satellite system and the ground system, and datais analyzed according to the time of generation of data. Timecorrelation between the satellite system and ground system can beestablished according to the time frame obtained from the satellitesystem and data can be analyzed using an accurate time. Therefore, thereis provided the advantage that it is possible to obtain a timesynchronizing method capable of acquiring a result of data analysisbased on accurate time in the satellite system.

In the present invention, processing for generation of a time frame tobe used for establishing a time correlation between the satellite timeand the ground system is executed, and then a transfer frame isgenerated by inserting, when transmitting the frames, this time frame atan arbitrary timing between the transmission frames. Time correlationbetween the satellite system and the ground system can be establishedaccording to this time frame by using a computer program. Thus, there isprovided the advantage that it is possible to obtain a recording mediumcontaining a computer program capable of precisely determined the timeof generation of data with a simple configuration.

In the present invention, when receiving the frames, time of generationof data in the satellite system is computed according to the time frameinserted between the received frames, to be used for establishing a timecorrelation between the satellite system and the ground system, and datais analyzed according to the computed time of generation of data. Datacan be analyzed according to the accurate time by establishing a timecorrelation between the satellite time and the ground time according tothe time frame obtained from the satellite system using a computerprogram. Thus, there is provided the advantage that it is possible toobtain a recording medium containing a computer program capable ofacquiring a result of analysis based on accurate time.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A time synchronization system comprising: asatellite system for transmitting a time frame by inserting the timeframe between transmission frames at a specified timing, wherein thetime frame comprises a frame containing information that indicates atime in the satellite; and a ground system for acquiring a time ofgeneration of data in the satellite system based on the informationindicating a time in the satellite in the time frame inserted betweenthe transmission frames; wherein the satellite system appends thesatellite time to data in the transmission frames when the data isgenerated in the satellite system, and wherein said satellite systemgenerates a satellite time by counting clocks synchronized to a bit rateof the transmission frames, and sets an entire portion below timerequired for transmitting one frame of the satellite time to zero in theheader of the transmission frame.
 2. A time synchronization systemcomprising: a satellite system for transmitting a time frame byinserting the time frame between transmission frames at a specifiedtiming, wherein the time frame comprises a frame containing informationthat indicates a time in the satellite; and a ground system foracquiring a time of generation of data in the satellite system based onthe information indicating a time in the satellite in the time frameinserted between the transmission frames; where the satellite systemappends the satellite time to data in the transmission frames when thedata is generated in the satellite system; where each of said equipmentshas an internal time, and said satellite system distributes a time clockto each of said equipments and synchronizes the internal time of each ofsaid equipment with the internal satellite time, where said satellitesystem inserts satellite time between transmission frames at twodifferent of timing, and where said ground system obtains an averagefrequency of the satellite time from a time interval between the twosatellite times inserted between received frames and from the timeinterval between the two ground times corresponding to the two satellitetimes, and corrects the time of generation of the data in said satellitesystem from this average frequency and an amount of drift from theofficial frequency of the satellite time.
 3. A time synchronizationsystem comprising: a satellite system for transmitting frames based oninternally generated data; a ground system for acquiring the datagenerated in said satellite system from the frames transmitted from saidsatellite system; wherein said satellite system generates a time framefor realizing time correlation between said satellite system and saidground system and inserts the generated time frame between thetransmission frames at an arbitrary timing, and said ground systemacquires the time of generation of the data in said satellite systemfrom the time frame inserted between received frames; wherein saidsatellite system appends the satellite time to the data generatedinternally, and wherein said satellite system generates the satellitetime by counting clocks synchronized to a bit rate of the transmissionframes, and sets an entire portion below time required for transmittingone frame of the satellite time to zero in the header of thetransmission frame.
 4. A time synchronization system according to claim3, wherein the satellite system generates the transmission frames bypacketizing the data generated in said satellite system.
 5. A timesynchronization system according to claim 3, wherein said satellitesystem comprises a plurality of equipments each generating a set ofobservation data comprising the internally generated data.
 6. A timesynchronization system according to claim 3, wherein said satellitesystem comprises a plurality of equipments each having an internal time,and said satellite system distributes a time clock to each of saidequipments and synchronizes an internal time of each of said equipmentwith the satellite time.
 7. A time synchronization system according toclaim 3, wherein said satellite system corrects the satellite timedepending upon a change in temperature.
 8. A time synchronization systemaccording to claim 3, wherein there are differing times betweengeneration of the generated data and transmission of frames containingthe generated data.
 9. A time synchronization system comprising: asatellite system for transmitting frames based on internally generateddata; a ground system for acquiring the data generated in said satellitesystem from the frames transmitted from said satellite system; whereinsaid satellite system generates a time frame for realizing timecorrelation between said satellite system and said ground system andinserts the generated time frame between the transmission frames at anarbitrary timing, and said ground system acquires the time of generationof the data in said satellite system from the time frame insertedbetween received frames; wherein said satellite system appends thesatellite time to the data generated internally; and wherein saidsatellite system inserts satellite time between transmission frames attwo different of timing, and wherein said ground system obtains anaverage frequency of the satellite time from a time interval between thetwo satellite times inserted between received frames and from the timeinterval between the two ground times corresponding to the two satellitetimes, and corrects the time of generation of the data in said satellitesystem from this average frequency and an amount of drift from theofficial frequency of the satellite time.
 10. A time synchronizationsystem according to claim 9, wherein said satellite system estimates anamount of change in the frequency to be used for acquiring the satellitetime depending upon a change in the internal temperature, and transmitsthe estimated amount of change in the frequency by inserting into thetransmission frame, while said ground system corrects the time ofgeneration of the data in said satellite system by taking into accountthe estimated amount of change in the frequency.
 11. A satellite systemapplied in a time synchronization system having a satellite system fortransmitting frames based on internally generated data, and a groundsystem for acquiring the data generated in said satellite system fromthe frames transmitted from said satellite system, said satellite systemcomprising: a time frame generator for generating a time frame to beused for establishing a time correlation between said satellite systemand said ground system; and a transfer frame generator for generatingtransfer frames by inserting a time frame generated by said time framegenerator between transmission frames at an arbitrary timing; whereinthe satellite time is appended to the data internally generated, whereinthe satellite time is generated by counting clocks synchronized to a bitrate of the transmission frames, and an entire portion of the satellitetime below the time required for transfer of one frame is set to zero.12. A satellite system according to claim 11, further comprising: aconvolution processor for generating the transfer frames by packetizingthe data generated in said satellite system.
 13. A satellite systemaccording to claims 11, further comprising a plurality of equipmentseach generating a set of observation data and said satellite systemgenerates the transmission frames based on this observation data.
 14. Asatellite system according to claim 13, wherein each of said equipmentshas an internal time, and said satellite system further has a timinggenerator for synchronizing the satellite time of said satellite systemwith the internal time of each of said equipments by distributing a timeclock to each of said equipments.
 15. A time synchronization systemaccording to claim 11, wherein there are differing times betweengeneration of the generated data and transmission of frames containingthe generated data.
 16. A satellite system according to claim 14,wherein the satellite time is corrected depending upon a change intemperature.
 17. A ground system applied in a time synchronizationsystem having a satellite system for transmitting frames based on aninternally generated data, and a ground system for acquiring the datagenerated in said satellite system from the frames transmitted from saidsatellite system; wherein said ground system determines a variable delaybetween a time of generation and a time of transmission of the datagenerated in said satellite system, when receiving the frames, based ona time correlation between a ground system time and a satellite systemtime of a frame inserted between the received frames; and wherein anaverage frequency is obtained from time interval between two differentsatellite times inserted between the transmission frames at twodifferent timing in the satellite system and from the time intervalbetween the ground times corresponding to the respective satellitetimes, and time of generation of the data in said satellite system iscorrected from this average frequency and from an official frequency ofthe satellite time.
 18. A ground system according to claim 17; whereinan amount of change in the frequency used for acquiring the satellitetime depending upon a change in the temperature estimated in saidsatellite system is taken into account when correcting the time ofgeneration of the data in said satellite system.
 19. A ground systemapplied in a time synchronization system having a satellite system fortransmitting frames based on an internally generated data, and a groundsystem for acquiring the data generated in said satellite system fromthe frames transmitted from said satellite system; wherein said groundsystem determines the time of generation of data in said satellitesystem when receiving the frames from the time frame to be used forestablishing a time correlation between the satellite system and theground system inserted between the received frames; and wherein anaverage frequency is obtained from time interval between two differentsatellite tiles inserted between the transmission frames at twodifferent timing in the satellite system and from the time intervalbetween the ground times corresponding to the respective satellitetimes, and time of generation of the data in said satellite system iscorrected from this average frequency and from an official frequency ofthe satellite time.